1. Field of the Invention
The present invention relates to an image emphasizing apparatus and an image emphasizing program for sharpening an image by emphasizing an outline of the image. Specifically, the present invention relates to an outline compensation circuit which is suitably used in various television receivers, various video apparatuses, such as a video tape recorder (VTR) or the like, and various image display apparatuses, such as liquid crystal displays, plasma displays, or the like.
2. Description of the Related Art
In a conventional television receiver, in order to recover a deterioration of a frequency characteristic of a video signal, which may be caused when an externally-supplied video signal is processed in various circuits before it is output to a display, or in order to improve the visual quality of an image which is reproduced from such a video signal, an outline compensation circuit is provided for performing outline compensation on the video signal. In the outline compensation circuit, a high-frequency component of the video signal (brightness signal) which is to be input to the display is extracted, and the extracted high-frequency component is added to the video signal, whereby rising and falling edges, which correspond to an outline of an image to be displayed, are sharpened. This conventional outline compensation circuit is described with reference to FIG. 17.
FIG. 17 is a circuit diagram showing an exemplary structure of the conventional outline compensation circuit. FIG. 18 shows signal waveforms which illustrate operations of respective sections of the outline compensation circuit shown in FIG. 17. The outline compensation circuit shown in FIG. 17 includes delay elements 87 and 88, adders 89, 90, 91, and 93, and a variable resistor 92 which is referred to as “sharpness volume”. Referring to FIGS. 17 and 18, in the outline compensation circuit, the delay element 87 outputs a signal Yb which has been obtained by delaying an externally-supplied video signal Ya. The delay element 88 outputs a signal Yc which has been obtained by delaying the signal Yb output from the delay element 87. The adder 89 outputs a difference signal Yd of the video signal Ya, which is input to the delay element 87, and the signal Yb, which is output from the delay element 87. The adder 90 outputs a difference signal Ye of the video signal Yb, which is input to the delay element 88, and the signal Yc, which is output from the delay element 88. The adder 91 adds the difference signal Yd, which is output from the adder 89, and the difference signal Ye, which is output from the adder 90, so as to output a signal Yf which corresponds to a secondary differentiation of the input video signal Ya. The variable resistor 92 adjusts the gain of the signal Yf output from the adder 91 and outputs the adjusted signal as an outline emphasizing signal Yg. The adder 93 adds the outline emphasizing signal Yg, which is output from the variable resistor 92, and the signal Yb, which is output from the delay element 87, so as to output a video signal Yh which expresses an image having an emphasized outline.
According to the above conventional outline compensation circuit, the video signal Yh, which expresses an image having an emphasized outline in comparison to an image reproduced from the input video signal Ya, is obtained. However, as shown in FIG. 18, in the video signal Yh output from the conventional outline compensation circuit, overshoot or undershoot occurs in a component of the video signal Yh which corresponds to an outline of an image to be displayed. If the overshoot or undershoot is large, the display image is deteriorated. That is, an image expressed by the output video signal Yh results in an unnatural image where the outline has a fringe.
An outline emphasizing apparatus disclosed in Japanese Laid-Open Publication No. 2000-244775 is structured such that the above problem is avoided. In this apparatus, a secondary differentiation signal which corresponds to a secondary differentiation of an input video signal is generated, and the secondary differentiation signal is compared with a predetermined threshold value. If the sign of the secondary differentiation signal is positive and the secondary differentiation signal is equal to or greater than the predetermined threshold value, an outline emphasizing signal is not added to the input video signal. Otherwise, the secondary differentiation signal is added as an outline emphasizing signal to the input video signal.
In the above outline emphasizing apparatus, a secondary differentiation signal is compared with a predetermined threshold value, and when the secondary differentiation signal is equal to or greater than the predetermined threshold value, the secondary differentiation signal is not added as an outline emphasizing signal to an input video signal. Due to such an arrangement, deterioration of the quality of a displayed image which is caused by the above-described overshoot or undershoot can be prevented. However, in such a method where overshoot and undershoot are suppressed by adding only a component of a secondary differentiation signal which is equal to or smaller than a predetermined threshold value, outline emphasization for a finely expressed portion of a displayed image, such as a thin line, a small spot, etc., is restricted. As a result, in the case of displaying an outline-compensated image on a high-definition display, such as a liquid crystal display, a plasma display, or the like, which have high definition, an improvement in the quality of a displayed image which is achieved by the above outline compensation method is small and insufficient.
This problem is described below with respect to FIG. 19.
Herein, assume that an input video signal Si has waveform W11 shown in FIG. 19. An outline component Sa extracted from the input video signal Si by secondary differentiation has a waveform W12 shown in FIG. 19. In the case where outline emphasization is performed by simply adding the outline component Sa to the input video signal Si, an output video signal Sc, which is obtained after the outline emphasization has been performed, has a waveform W13 shown in FIG. 19. In this case, as previously described, overshoot or undershoot which occurs in a signal component of the output video signal Sc, which corresponds to an outline of a displayed image, becomes large. A known method for avoiding such a problem is a method for suppressing overshoot or undershoot, which may occur through an outline emphasization process, based on a predetermined threshold value. Now, based on such a method, assume an example where an excessive emphasis correction threshold value α (>0) is employed, and an outline component Sa is compared with the excessive emphasis correction threshold value α and with a value obtained by inverting the excessive emphasis correction threshold value α (hereinafter, referred to as an “inverted excessive emphasis correction threshold value”). A component of the outline component Sa which exceeds a range of −α to α is not added to the input video signal Si, but only a component within the range of −α to α is added to the input video signal Si. (Hereinafter, such an outline compensation example is simply referred to as a “conventional example”.) That is, in this conventional example, the amplitude (in absolute value) of an outline emphasizing signal Ss which is to be added to the input video signal Si is, as shown by a solid line portion of a waveform W14 of FIG. 19, restricted by the excessive emphasis correction threshold value α. The output video signal Sc has a waveform W15 shown in FIG. 19. In this way, according to such a conventional example, outline emphasization is performed only with a component of the outline component Sa within the range of −α to α, whereby overshoot and undershoot can be suppressed, while an inclination of a signal component which corresponds to an outline of an input video signal is increased.
However, in this conventional example, in the case where the input video signal Si has a waveform W16 shown in FIG. 19, the outline emphasizing signal Ss results in a signal represented by a solid line portion of a waveform W17. (Note that in FIG. 19, a dotted line waveform W18 represents an outline component signal.) As a result, the output video signal Sc results in a waveform W19 shown in FIG. 19. The waveform W19 indicates that, in the conventional outline compensation example, for a high-definition image including thin lines, small spots, etc., an improvement in the quality of the displayed high-definition image is insufficient.